The present disclosure relates to the design of electromagnetic shielding and lightning strike protection. More particularly, the present disclosure relates to polymeric, elastomeric, and ceramic materials, and to composites in which such materials are employed as an adhesive, a matrix, or a coating, and to surfaces of static or mobile assemblies in which are incorporated into such materials or composites employing such materials. Such materials and composites relate to the enhancement of the electrical conductivity of such materials, composites, and surfaces, thereby to afford lightning strike protection to assemblies in which such materials or such composites are incorporated.
Polymeric materials, either alone or as composites reinforced with powders or fibers, are attractive engineering compositions, exhibiting characteristics of cost, weight, and workability that individually or in combination prove decisively advantageous on behalf of use in diverse circumstances.
For example, polymeric materials and fiber-reinforced polymer-based composites are acquiring increased importance in the construction of fixed assemblies, such as outdoor shelters, signal antennas, power transmission towers, and wind generators, as well as in the construction of mobile assemblies, such as land vehicles, ships, aircraft, satellites, and rockets. In constructing these types of assemblies, a combination of light weight and remarkable stiffness makes polymeric materials and polymer-based composites having reinforcing fibers superior to metal as compositions from which to fashion components.
Yet, in most cases, polymeric materials and polymer-based composites lack the electromagnetic responsiveness associated with metals. Even when a polymer-based composite includes reinforcing fibers that are electrically-conductive, as is the case with carbon fibers and with metal-coated fibers, the resulting composite is electrical conductive only in a direction that is aligned with the fibers embedded therein.
Poor electrically conductivity in polymeric materials and polymer-based composites is problematic when components fashioned therefrom are incorporated into the surface of fixed or mobile assemblies.
Surface locations wanting in an ability to conduct electricity allow localized electrostatic surface charges to build up over time. These charges are then capable of dissipation only through the potentially incendiary and surface-damaging mechanism of sparking and arcing.
On an electrically-nonconductive surface of an assembly, the electricity from a lightning strike is not conducted safely from the site of the strike along or through the surface to intended grounding structures. Consequently, notorious forms of disintegration befall the assembly at the site of the strike. There, blistering, melting, delamination, incineration, and even penetration are common.
Accompanying a lightning strike are intense spikes of electromagnetic energy that induce correspondingly large pulses of electrical current in electrical circuitry in the vicinity of the lightning, both disrupting and disabling nearby electrical circuit elements. Onboard navigation and control systems on aircraft, rockets, and satellites are particular vulnerable to such electromagnetic damage, whether or not originating from lightning, and devastating secondary consequences following when such systems fail. Misfortunes of such origin can be mitigated or avoided entirely by surrounding vulnerable circuitry with an appropriately-configured shield that either absorbs or reflects incoming electromagnetic energy.
Any such electromagnetically-impenetrable shield must, however, be highly and solidly electrically-conductive, as would be the case with an unbroken metal skin on the surface of an assembly in which vulnerable circuitry is deployed. Accordingly, the replacement of metal in surfaces of fixed and mobile assemblies by polymeric, elastomeric, and ceramic materials and by fiber composites in which such materials are employed eliminates advantageous electromagnetic shielding for circuits within those assemblies.
Numerous attempts have been made to overcome or ameliorate the want of electrical conductivity in surfaces of assemblies that incorporate polymeric, elastomeric, and ceramic materials and fiber composites in which such materials are employed.
One general approach to restoring electrical conductivity, involves adding to the outer surface of an electrically-nonconductive base structure a ground plane of electrically-conductive broad goods, such as wires, screens, meshes, weavings, or expanded foils made of metals, such as copper, brass, or aluminum. In the alternative, the broad goods in such a ground plane can include electrically-nonconductive fibers that have been rendered electrically-conductive by being electroplated or otherwise coated with an electrically-conductive material, such as a metal. Electrically-nonconductive fibers coated with metal are also woven into fabrics or bound into papers before being employed in ground planes as electrically-conductive broad goods. Wire weave fabrics made of metal fibers interwoven with electrically-nonconductive fabrics also serve as electrically-conductive broad goods in ground planes.
Another general approach to restoring electrical conductivity involves making the outer layer of the electrically-nonconductive base structure into a layer that itself exhibits electrically-conductivity. This can be accomplished, for example, by incorporating wound or woven wire or nickel-coated carbon fibers into the outer plies of the base structure. Alternatively, electrically-conductive additives are introduced into the polymeric material from which those outer plies are eventually fashioned. Acceptable additives for this purpose include powders of metals, such as silver, copper, nickel, or iron, and fibers made of or coated with such metals.
Yet a final general approach to restoring electrical conductivity involves applying to the outer surface of the electrically-nonconductive base structure electrically-conductive foils or a coating of a paint containing flakes, particles, or powders of metals, such as silver or copper.
Each approach described above has demonstrated advantages and demonstrated shortcomings, but no single approach or combination of approaches has enabled an electrically-nonconductive composite article incorporated into the surface of a fixed or mobile assembly to fully withstand a lightning strike both, by efficiently dissipating the energy of the lightning strike, and by shielding the interior of the assembly for electromagnetic pulses associated with the lightning strike.